/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.collect;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Objects;
import java.util.Map;
import java.util.Map.Entry;
import javax.annotation.Nullable;

/**
 * A map entry which forwards all its method calls to another map entry. Subclasses should override
 * one or more methods to modify the behavior of the backing map entry as desired per the
 * <a href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
 *
 * <p>
 * <b>Warning:</b> The methods of {@code ForwardingMapEntry} forward <i>indiscriminately</i> to the
 * methods of the delegate. For example, overriding {@link #getValue} alone <i>will not</i> change
 * the behavior of {@link #equals}, which can lead to unexpected behavior. In this case, you should
 * override {@code equals} as well, either providing your own implementation, or delegating to the
 * provided {@code standardEquals} method.
 *
 * <p>
 * Each of the {@code standard} methods, where appropriate, use {@link Objects#equal} to test
 * equality for both keys and values. This may not be the desired behavior for map implementations
 * that use non-standard notions of key equality, such as the entry of a {@code SortedMap} whose
 * comparator is not consistent with {@code equals}.
 *
 * <p>
 * The {@code standard} methods are not guaranteed to be thread-safe, even when all of the methods
 * that they depend on are thread-safe.
 *
 * @author Mike Bostock
 * @author Louis Wasserman
 * @since 2.0
 */
@GwtCompatible
public abstract class ForwardingMapEntry<K, V> extends ForwardingObject implements Map.Entry<K, V> {
    // TODO(lowasser): identify places where thread safety is actually lost

    /** Constructor for use by subclasses. */
    protected ForwardingMapEntry() {}

    @Override
    protected abstract Map.Entry<K, V> delegate();

    @Override
    public K getKey() {
        return delegate().getKey();
    }

    @Override
    public V getValue() {
        return delegate().getValue();
    }

    @Override
    public V setValue(V value) {
        return delegate().setValue(value);
    }

    @Override
    public boolean equals(@Nullable Object object) {
        return delegate().equals(object);
    }

    @Override
    public int hashCode() {
        return delegate().hashCode();
    }

    /**
     * A sensible definition of {@link #equals(Object)} in terms of {@link #getKey()} and
     * {@link #getValue()}. If you override either of these methods, you may wish to override
     * {@link #equals(Object)} to forward to this implementation.
     *
     * @since 7.0
     */
    protected boolean standardEquals(@Nullable Object object) {
        if (object instanceof Entry) {
            Entry<?, ?> that = (Entry<?, ?>) object;
            return Objects.equal(this.getKey(), that.getKey()) && Objects.equal(this.getValue(), that.getValue());
        }
        return false;
    }

    /**
     * A sensible definition of {@link #hashCode()} in terms of {@link #getKey()} and
     * {@link #getValue()}. If you override either of these methods, you may wish to override
     * {@link #hashCode()} to forward to this implementation.
     *
     * @since 7.0
     */
    protected int standardHashCode() {
        K k = getKey();
        V v = getValue();
        return ((k == null) ? 0 : k.hashCode()) ^ ((v == null) ? 0 : v.hashCode());
    }

    /**
     * A sensible definition of {@link #toString} in terms of {@link #getKey} and {@link #getValue}.
     * If you override either of these methods, you may wish to override {@link #equals} to forward
     * to this implementation.
     *
     * @since 7.0
     */
    @Beta
    protected String standardToString() {
        return getKey() + "=" + getValue();
    }
}
